The subject invention is directed to a welding process for drawn arc stud welding, as well as a drawn arc stud welding device for execution of the process.
In the well known drawn arc stud welding process, the part which is to be welded together with a work piece is initially placed on the work piece by means of a welding head, which can be designed as a stud welding gun. After switching on a pre-sparking or pilot current of approximately 10 to 100 A, the part is lifted off the work piece, so that a pilot electric arc is formed. Following the drawing of the pilot electric arc, the amperage is increased by a multiple up to the current level for the main current electric arc (I&gt;100 A). After ignition of the main current electric arc, and following observance of a predetermined welding waiting time, the part is moved toward the melted work piece surface and plunged into the melted puddle. After plunging, the welding current is turned off and welding is finished.
In practical application, however, it has been shown that with stud welding, weldings of unsatisfactory quality occur particularly when there are impurities on the surfaces of the parts which are to be welded together. This involves mostly oil or grease residues on the work piece, which may, for example, be sheet plate produced by a drawing process, that is coated with a drawing lubricant.
Furthermore, similar problems occur with welding of coated, for example galvanized, parts. The presence of rust or a primer on the surfaces to be welded can likewise result in detrimental influences on the welding quality.
Therefore, it has also been known for quite some time to superimpose on the pilot arc pulses with high peak current intensities, in order to burn off impurities which may be present on the surfaces of the parts which are to be welded together (see, for example, U.S. Pat. No. 3,496,325).
It was, however, considered as a drawback, that the purification pulses were activated either manually or automatically with each welding, independent of the quality of the surfaces to be welded together.
For that reason, the control device for stud welding described in European Patent EPO 241 249 was developed. A pilot arc monitoring voltage is converted to a correction voltage by means of a set-point comparator and is superimposed on a control voltage for principally controlling a high-frequency modulated switching mode power supply unit, in accordance with the thus corrected control voltage, that adjusts its output current during the respective welding operation to the resistance value of the pilot arc as established by the monitoring voltage. In this fashion, recognition of the quality of the surfaces to be welded is achieved through measuring of the voltage of the pilot arc (as was already known from German Patent 3130389C2). Thereafter, cleaning of surfaces can be obtained through an increase of the pilot current for a given period of time.
This process in essence is based upon the teaching from German Patent 3130389C2, that a heavily soiled, for example greasy surface, exercises a clearly measurable influence upon the voltage of the pilot arc, while the voltage of the main current electric arc remains practically unaffected thereby.
Since with these known welding processes the attained quality of the finished welding is to be appraised based on the voltage of the pilot arc, relative uncertainty results in the evaluation of the obtained welding quality, inasmuch as it is true that inadequately prepared or soiled surfaces are being recognized, but it is not possible to control the actually achieved improvement through altering of the welding current (pilot current--and/or the main current electric arc) and/or the activation of purification pulses.
Proceeding from the above-discussed state of the art, the invention is based on the objective to create another welding process for drawn arc stud welding, which makes it possible to recognize inadequately prepared surfaces to be welded and which reduces, through appropriate measurements, their impact upon the quality of the finished welding. In addition, the invention is directed to a device for the execution of the process.